Lessons from studies of impact crater hydrothermal processes in terrestrial analogs and their implications for impact craters on Mars

Studying hydrothermal processes in terrestrial impact craters as martian analogs has sometimes been fraught with objections, including the Earth's greater abundance of water, the neutral instead of acidic aqueous environments and the composition of the targets. Although recent discoveries have dispelled many objections, some misconceptions remain. For example, the relevance of the Chicxulub crater as a martian analog is sometimes questioned because the target was covered with sediments, including carbonates and sulfates. However the impactites at the Yaxcopoil-1 drill site are derived from the underlying silicate basement. Comparisons can also be difficult because of scale issues, as many terrestrial craters with evidence of hydrothermal activity, e.g. Lonar, Haughton, Ries etc., are smaller than the Martian craters with phyllosilicate signatures (Ehlmann et al., 2010). Summarizing, the results of many studies of terrestrial craters show that: 1) Most terrestrial craters larger than 1.8 km diameter have at least some evidence of aqueous or hydrothermal processes in the form of alteration minerals (e.g., Naumov, 2005). 2) Impact melts in crater fill and ejecta blankets provide heat that can produce hydrothermal alteration if water is available (Newsom, 1980). 3) The uplifted geothermal gradient can be as important a heat source as shock effects. 4) Mineralogical evidence for high-temperature fluids (> 350 ^oC) is present in the central uplift of the Manson structure, and in the ejecta from the Chicxulub impact, where precipitation of phyllosilicates from hydrothermal fluids has also been described (Newsom et al., 2010). 5) Impact deposits begin hot, but have an extended cooling period during which alteration phases can back react to low temperature phases with corresponding stable isotope signatures. 5) Hydrothermal fluids can travel long distances from their sources (e.g., Chicxulub, Yaxcopoil site) and are often localized to faults or porous breccias (e.g. Sudbury), producing alteration zones that are spatially limited. For Mars therefore, where will evidence of impact hydrothermal processes be found? A) Extensive hydrothermal alteration requires large craters (>20 km diam.), with heat from basement uplift, and the presence of shocked and melted material in crater fill and ejecta. B) Assuming water is available from precipitation, ice, or groundwater, hydrothermal fluids can be generated in impact melt sheets, melt-bearing ejecta, and central uplifts. C) Hydrothermal fluids can contribute to the formation of impact crater lakes with accompanying precipitation of evaporites and alteration of materials on the lake floors. D) Hydrothermal fluids derived from hot central uplifts and melt sheets may also migrate into porous megabreccias and faulted rocks associated with crater walls and central uplifts leading to formation or precipitation of alteration minerals. C) Outside of large craters or basins, alteration of melt-bearing ejecta can occur if the ejecta is relatively thick (> 100 m), and water is available. In conclusion, based on terrestrial analog studies, impact hydrothermal processes are a plausible explanation for the alteration phases observed in association with Martian craters.